Selective imidazoline I2 ligands do not show antidepressant-like activity in the forced swim test in mice

Sex differences in the antidepressant-like response and molecular events induced by the imidazoline-2 receptor agonist CR4056 in rats

In searching for novel targets to design antidepressants, among the characterized imidazoline receptors (IR), I2 receptors are an innovative therapeutical approach since they are dysregulated in major depressive disorder and by classical antidepressant treatments. In fact, several I2 agonists have been characterized for their antidepressant-like potential, but the results in terms of efficacy were mixed and exclusively reported in male rodents. Since there are well-known sex differences in antidepressant-like efficacy, this study characterized the potential effects induced by two I2 drugs, CR4056 (i.e., most promising drug already in phase II clinical trial for its analgesic properties) and B06 (a compound from a new family of bicyclic α-iminophosphonates) under the stress of the forced-swim test in male and female rats exposed to early-life stress. Moreover, some hippocampal neuroplasticity markers related to the potential effects observed were also evaluated (i.e., FADD, p-ERK/ERK, mBDNF, cell proliferation: Ki-67 + cells). The main results replicated the only prior study reporting the efficacy of CR4056 in male rats, while providing new data on its efficacy in females, which was clearly dependent on prior early-life stress exposure. Moreover, B06 showed no antidepressant-like effects in male or female rats. Finally, CR4056 increased FADD content and decreased cell proliferation in hippocampus, without affecting p-ERK/t-ERK ratio and/or mBDNF content. Interestingly, these effects were exclusively observed in female rats, and independently of early-life conditions, suggesting some distinctive molecular underpinnings participating in the therapeutic response of CR4056 for both sexes. In conjunction, these results present CR4056 with an antidepressant-like potential, especially in female rats exposed to stress early in life, together with some neuronal correlates described in the context of these behavioral changes in females.

Linalool as a Therapeutic and Medicinal Tool in Depression Treatment: A Review

Depression is a prevalent disease worldwide, limiting psychosocial functioning and thequality of life. Linalool is the main constituent of some essential oils from aromatic plants, representing about 70% of these volatile concentrates. Evidence of the linalool activity on the central nervous system, mainly acting as an antidepressant agent, is increasingly abundant. This review aimed to extend the knowledge of linalool's antidepressant action mechanisms, which is fundamental for future research, intending to highlight this natural compound as a new antidepressant phytomedication. A critical analysis is proposed here with probable hypotheses of the synergic mechanisms that support the evidence of antidepressant effects of the linalool. The literature search has been conducted in databases for published scientific articles before December 2020, using relevant keywords. Several pieces of evidence point to the anticonvulsant, sedative, and anxiolytic actions. In addition to these activities, other studies have revealed that linalool acts on the monoaminergic and neuroendocrine systems, inflammatory process, oxidative stress, and neurotrophic factors, such as BDNF, resulting in considerable advances in the knowledge of the etiology of depression. In this context, linalool emerges as a promising bioactive compound in the therapeutic arsenal, capable of interacting with numerous pathophysiological factors and acting on several targets. This review claims to contribute to future studies, highlighting the gaps in the linalool knowledge, such as its kinetics, doses, routes of administration, and multiple targets of interaction, to clarify its antidepressant activity.

Are Noradrenergic Transmission Reducing Drugs Antidepressants?

Major depressive disorder (MDD) remains a significant public health problem worldwide, and revised treatment strategies are therefore urgently needed, including the creation of novel antidepressant compounds or using existing molecular entities in new ways. Etiologic theories of MDD from decades ago have suggested that synaptic deficiencies of monoaminergic neurotransmitters play a causative role in this neuropsychiatric disorder, and that boosting monoamines with drugs such as SSRIs, SNRIs, TCAs, and MAOIs has antidepressant effects and in some individuals can even induce hypomania or mania. While other factors, such as various intracellular molecular pathways and hippocampal neurogenesis, undoubtedly also play a role in MDD, monoaminergic boosting drugs nonetheless have clearly demonstrated antidepressant properties. There is also, however, a body of studies in the preclinical literature suggesting that monoaminergic transmission reducing drugs, including noradrenergic ones, also have antidepressant-like behavioral properties in rodents. Given that there is increasing evidence that the monoamines have u-shaped or Janus-faced dose-response properties, in which a mid-range value is "optimal" in a variety of behavioral and physiological processes, it is plausible that either too much or too little synaptic norepinephrine in key circuits may exacerbate MDD in some individuals. Here we briefly review rodent depression-related behavioral data, focusing on the forced swim test, from three major classes of noradrenergic transmission reducing drugs (alpha2 agonists, beta blockers, alpha1 antagonists), and find much support for the hypothesis that they have antidepressant-like properties. Whether these drugs are antidepressants in human subjects remains to be determined.

Evaluation of the neuropharmacological effects of Gardenin A in mice

This work describes the neuropharmacological (sedative, anxiolytic, antidepressant, and anticonvulsant) actions of Gardenin A (GA) (0.1-25 mg/kg p.o.), a flavonoid found in medicinal plants. The sedative effects of GA were assessed with the pentobarbital-induced sleep test. The anxiolytic actions of GA were evaluated with the elevated plus-maze, the light-dark box test, the exploratory cylinder assay, and the open field test. Motor coordination was evaluated with the rotarod test and the open field test. The antidepressant-like actions of GA were evaluated with the tail suspension test and forced swimming test. The mechanisms of the anxiolytic-like and antidepressant-like effects of GA were assessed using inhibitors of neurotransmission pathways. The anticonvulsant activity of GA was evaluated with the strychnine-induced seizure test. The sedative effects of GA were evident only at a dose of 25 mg/kg, which increased the duration of sleep but did not alter sleep onset. GA showed anxiolytic-like actions with activity comparable to that of clonazepam in all experimental tests. The GABA_A receptor antagonist bicuculline reversed the anxiolytic-like effects of GA. Furthermore, GA showed significant antidepressant-like actions in both models with activity comparable to that of fluoxetine. Yohimbine, an α2-adrenoceptor blocker, inhibited the antidepressant-like actions of GA. In addition, GA (1-10 mg/kg) did not affect locomotor coordination in mice and delayed the onset of convulsions. These findings suggest that GA induces anxiolytic-like effects and has anticonvulsant actions by the possible involvement of the GABAergic system. The antidepressant-like actions of GA may be mediated by noradrenergic neurotransmission.

The α2C-adrenoceptor antagonist, ORM-10921, exerts antidepressant-like effects in the Flinders Sensitive Line rat

Depression involves deficits in monoaminergic neurotransmission. Differential roles for α2A, B and C subtypes of the α2-adrenoceptor (AR) are evident, with selective α2C-AR antagonists purported to have antidepressant and procognitive properties. However, this has not been demonstrated in a genetic animal model of depression. The role of the α2C-AR in modulating two key depression-related behaviours in the Flinders Sensitive Line (FSL) rat was studied using a dose-response analysis following subcutaneous administration with the selective α2C-AR antagonist ORM-10921 (0.03; 0.3 mg/kg), the nonselective α2-AR antagonist idazoxan (3 mg/kg), or vehicle once daily for 14 days. Behaviour in the novel object recognition test, forced swim test (FST) and locomotor activity test was assessed. To ratify the validity of the FSL model, the reference tricyclic antidepressant imipramine (15 mg/kg, intraperitoneally) was used as a comparator drug in the FST. FSL rats demonstrated significantly increased immobility and recognition memory deficits versus Flinders Resistant Line controls, with imipramine significantly reversing said immobility. Similarly, ORM-10921 at both doses but not idazoxan significantly reversed immobility in the FST as well as attenuated cognitive deficits in FSL animals. We conclude that selective α2C-AR antagonism has potential as a novel therapeutic strategy in the treatment of depression and cognitive dysfunction.

Biphasic Effects of α-Asarone on Immobility in the Tail Suspension Test: Evidence for the Involvement of the Noradrenergic and Serotonergic Systems in Its Antidepressant-Like Activity

Alpha (α)-asarone is one of the main psychoactive compounds, present in Acorus species. Evidence suggests that the α-asarone possess an antidepressant-like activity in mice. However, the exact dose-dependent effect of α-asarone and mechanism(s) involved in the antidepressant-like activity are not clear. The present study aimed to investigate the dose-dependent effect of α-asarone and the underlining mechanism(s) involved in the antidepressant-like activity of α-asarone in the mouse model of tail suspension test (TST). In this study, the acute effect of α-asarone per se at different doses (10–100 mg/kg, i.p.) on immobility in the TST was studied. Additionally, the possible mechanism(s) involved in the antidepressant-like effect of α-asarone was studied using its interaction with noradrenergic and serotonergic neuromodulators in the TST. The present results reveal that the acute treatment of α-asarone elicited biphasic responses on immobility such that the duration of the immobility time is significantly reduced at lower doses (15 and 20 mg/kg, i.p.) but increased at higher doses (50 and 100 mg/kg, i.p.) in the TST. Besides, α-asarone at higher doses (50 and 100 mg/kg, i.p.) significantly decreased the spontaneous locomotor activity. Moreover, pretreatment of mice with noradrenergic neuromodulators such as AMPT (100 mg/kg, i.p., a catecholamine synthesis inhibitor), prazosin (1 mg/kg, i.p., an α₁-adrenoceptor antagonist), yohimbine (1 mg/kg, i.p., an α₂-adrenoceptor antagonist) and with serotonergic neuromodulators such as PCPA (100 mg/kg, i.p., once daily for four consecutive days, a serotonin synthesis inhibitor,) and WAY100635 (0.1 mg/kg, s.c., a selective 5-HT_1A receptor antagonist) significantly reversed the anti-immobility effect of α-asarone (20 mg/kg, i.p.). Taken together, our results suggest that the acute treatment with α-asarone elicited biphasic actions in the TST in which antidepressant-like effect was seen at relatively lower doses (15 and 20 mg/kg, i.p.) and depressive-like activity at relatively higher doses (50 and 100 mg/kg, i.p.). Furthermore, it has been revealed that the antidepressant-like effect of α-asarone could be mediated through both noradrenergic (α₁ and α₂ adrenoceptors) and serotonergic (particularly, 5-HT_1A receptors) systems.

Management of bipolar depression with lamotrigine: an antiepileptic mood stabilizer

The efficacy of lamotrigine in the treatment of focal epilepsies have already been reported in several case reports and open studies, which is thought to act by inhibiting glutamate release through voltage-sensitive sodium channels blockade and neuronal membrane stabilization. However, recent findings have also illustrated the importance of lamotrigine in alleviating the depressive symptoms of bipolar disorder, without causing mood destabilization or precipitating mania. Currently, no mood stabilizers are available having equal efficacy in the treatment of both mania and depression, two of which forms the extreme sides of the bipolar disorder. Lamotrigine, a well established anticonvulsant has received regulatory approval for the treatment and prevention of bipolar depression in more than 30 countries worldwide. Lamotrigine, acts through several molecular targets and overcomes the major limitation of other conventional antidepressants by stabilizing mood from “below baseline” thereby preventing switches to mania or episode acceleration, thus being effective for bipolar I disorder. Recent studies have also suggested that these observations could also be extended to patients with bipolar II disorder. Thus, lamotrigine may supposedly fulfill the unmet requirement for an effective depression mood stabilizer.

Linalool and β-pinene exert their antidepressant-like activity through the monoaminergic pathway

AIMS

Linalool and β-pinene are two volatile monoterpenes that possess antidepressant-like activity. These are components of many aromatic plants used in folk medicine around the world to relieve anxiety and depression. In this contribution, we focused on examining the mechanism of action of these compounds.

MAIN METHODS

We used mice in the forced swimming test (FST) and antagonist drugs (i.p.) to receptors related to the depression process such as 5-HT1A. To assess the possible contribution of the serotoninergic system, animals were pre-treated with WAY 100635 (a 5-HT1A receptor antagonist) and PCPA (a serotonin synthesis inhibitor).To assess the participation of the noradrenergic system, the animals were pre-treated with yohimbine (an α2 receptor antagonist), propranolol (a β receptor antagonist) and neurotoxin DSP-4 (a noradrenergic neurotoxin). In the dopaminergic system, we used SCH23390 (a D1 receptor antagonist).

KEY FINDINGS

WAY 100635 blocked the antidepressant-like effect of linalool and β-pinene. In contrast, pretreatment of mice with PCPA did not modify reductions in the immobility time elicited by the two monoterpenes. The yohimbine modified the effect of linalool on immobility time. Propranolol and neurotoxin DSP-4 reversed the anti-immobility effect of β-pinene; also, SCH23390 blocked the antidepressant-like effect of β-pinene.

SIGNIFICANCE

Our results indicate that linalool and β-pinene produce an antidepressant-like effect through interaction with the monoaminergic system.

Gender difference in epileptogenic effects of 2-BFI and BU224 in mice

Imidazoline I2 receptors are involved in pain modulation and psychiatric disorders and its ligands may represent a new therapeutic strategy against pain and depression. In particular, 2-BFI and BU224 are the two most widely studied I2 receptor ligands and have antinociceptive and antidepressant-like activities in rodents. However, little is known of the toxicological effects and potential gender differences of these I2 receptor ligands. This study examined the epileptogenic activities of 2-BFI and BU224 in male and female mice and also examined their underlying receptor mechanisms. 2-BFI (10-40 mg/kg, i.p.) and BU224 (10-40 mg/kg) produced epileptic seizures in a dose-related manner, as did the epileptogenic agent, pentylenetetrazole (PTZ, 15-60 mg/kg). However, female mice were significantly more sensitive than male mice in all the measures. The commonly used I2 receptor antagonist, idazoxan (10mg/kg), did not block the onset and magnitude of the epileptic seizures or lethality induced by 2-BFI and BU224. When studied in combination, PTZ potentiated the epileptogenic effect of 2-BFI and BU224. The lack of antagonism by idazoxan of the epileptogenic activities of 2-BFI and BU224 suggests that the epileptogenic effects of 2-BFI and BU224 are mediated by non-imidazoline I2 receptors and that I2 receptors remain a viable therapeutic target for neurological disorders such as pain.

Characterization of the hypothermic effects of imidazoline I₂ receptor agonists in rats

BACKGROUND AND PURPOSE Imidazoline I(2) receptors have been implicated in several CNS disorders. Although several I(2) receptor agonists have been described, no simple and sensitive in vivo bioassay is available for studying I(2) receptor ligands. This study examined I(2) receptor agonist-induced hypothermia as a functional in vivo assay of I(2) receptor agonism. EXPERIMENTAL APPROACH Different groups of rats were used to examine the effects of I(2) receptor agonists on the rectal temperature and locomotion. The pharmacological mechanisms were investigated by combining I(2) receptor ligands and different antagonists. KEY RESULTS All the selective I(2) receptor agonists examined (2-BFI, diphenyzoline, phenyzoline, CR4056, tracizoline, BU224 and S22687, 3.2-56 mg·kg(-1) , i.p.) dose-dependently and markedly decreased the rectal temperature (hypothermia) in rats, with varied duration of action. Pharmacological mechanism of the observed hypothermia was studied by combining the I(2) receptor agonists (2-BFI, BU224, tracizoline and diphenyzoline) with imidazoline I(2 ) receptor/ α(2) adrenoceptor antagonist idazoxan, selective I(1) receptor antagonist efaroxan, α(2) adrenoceptor antagonist/5-HT(1A) receptor agonist yohimbine. Idazoxan but not yohimbine or efaroxan attenuated the hypothermic effects of 2-BFI, BU224, tracizoline and diphenyzoline, supporting the I(2) receptor mechanism. In contrast, both idazoxan and yohimbine attenuated hypothermia induced by the α(2) adrenoceptor agonist clonidine. Among all the I(2) receptor agonists studied, only S22687 markedly increased the locomotor activity in rats. CONCLUSIONS AND IMPLICATIONS Imidazoline I(2) receptor agonists can produce hypothermic effects, which are primarily mediated by I(2) receptors. These data suggest that I(2) receptor agonist-induced hypothermia is a simple and sensitive in vivo assay for studying I(2) receptor ligands.

Involvement of monoaminergic systems in the antidepressant-like effect of Eugenia brasiliensis Lam. (Myrtaceae) in the tail suspension test in mice

ETHNOPHARMACOLOGICAL RELEVANCE

Several species of Eugenia L. are used in folk medicine for the treatment of various diseases. Eugenia brasiliensis is used for the treatment of inflammatory diseases, whereas Eugenia. uniflora is used for the treatment of symptoms related to depression and mood disorders, and is used in Brazil by the Guarani Indians as a tonic stimulant.

AIM OF THE STUDY

To investigate the antidepressant-like effect of hydroalcoholic extracts of different plant species of genus Eugenia and to characterize the participation of the monoaminergic systems in the mechanism of action of the specie that afforded the most prominent antidepressant-like efficacy.

MATERIALS AND METHODS

In the first set of experiments, the effects of hydroalcoholic extracts of Eugenia beaurepaireana, Eugenia brasiliensis, Eugenia catharinae, Eugenia umbelliflora and Eugenia uniflora and the antidepressant fluoxetine (positive control) administered acutely by p.o. route were evaluated in the tail suspension test (TST) and locomotor activity was assessed in the open-field test in mice. In the second set of experiments, the involvement of the monoaminergic systems in the antidepressant-like activity of Eugenia brasiliensis was evaluated by treating mice with several pharmacological agonists and antagonists. The effects of the combined administration of sub-effective doses of Eugenia brasiliensis and the antidepressants fluoxetine, imipramine and bupropion were also evaluated.

RESULTS

The administration of the extracts from Eugenia brasiliensis, Eugenia catharinae and Eugenia umbelliflora, but not Eugenia beaurepaireana and Eugenia uniflora, exerted a significant antidepressant-like effect, without altering locomotor activity. The behavioral profile was similar to fluoxetine. Pre-treatment of mice with ketanserin, haloperidol, SCH23390, sulpiride, prazosin and yohimbine prevented the reduction of immobility time induced by Eugenia brasiliensis. Treatment with sub-effective doses of WAY100635, SKF38393, apomorphine, phenylephrine, but not clonidine, combined with a sub-effective dose of Eugenia brasiliensis decreased the immobility time in the TST. Furthermore, the combined administration of sub-effectives doses of Eugenia brasiliensis with fluoxetine, imipramine and bupropion produced an antidepressant-like effect.

CONCLUSIONS

This study show, for the first time, the antidepressant-like effect of species of the genus Eugenia, especially Eugenia brasiliensis, whose effects in the TST seem to be mediated by serotoninergic (5-HT(1A) and 5-HT(2) receptors), noradrenergic (α(1)-adrenoceptor) and dopaminergic (dopamine D(1) and D(2) receptors) systems.

The potential antidepressant-like effect of imidazoline I2 ligand 2-BFI in mice

The compound 2-(2-benzofuranyl)-2-imidazoline (2-BFI) is a 2-imidazoline derivative that selectively inhibits the in vitro activity of monoamine oxidase-A and it is also an imidazoline I(2) agonist. However, the antidepressant potential of this compound and its mechanism of action have not been well defined. Therefore, in this study we investigated the antidepressant-like effect of 2-BFI in mice. 2-BFI (100 and 300μmol/kg, s.c.) significantly reduced the immobility time on the tail suspension test (TST) without changing locomotion in the open field test. The reduced the immobility time of 2-BFI (100μmol/kg, s.c.) was confirmed with the forced swimming test (FST). The antidepressant-like effect of 2-BFI (100μmol/kg, s.c.) in the TST was prevented by pretreatment with idazoxan (0.4μmol/kg, i.p., a I(2) site antagonist), methysergide (4μmol/kg, i.p., a non-selective serotonergic receptor antagonist) and haloperidol (0.1μmol/kg, i.p., a non-selective dopaminergic receptor antagonist). The anxiolytic effect of 2-BFI was also evaluated, using the elevated plus-maze test. 2-BFI (300μmol/kg, s.c.) was able to significantly increase the % of number of entries and the % of time spent in the open arms, indicating that it possesses an anxiolytic effect at high doses. In conclusion, these results suggest that the antidepressant-like effect of 2-BFI might involve serotonergic, dopaminergic and imidazoline systems, and then the imidazoline site could represent a new pharmacological target for the treatment of depression.

Noradrenergic antidepressant responses to desipramine in vivo are reciprocally regulated by arrestin3 and spinophilin

Many antidepressant drugs, including the tricyclic antidepressant desipramine (DMI), are broadly understood to function by modulating central noradrenergic neurotransmission. α(2) adrenergic receptors (α(2)ARs) are key regulators of the noradrenergic system, and previous work has implicated α(2)ARs in mediating the antidepressant activity of DMI in the rodent forced swim test (FST). However, little is known about intracellular regulators of antidepressant drug action. α(2)AR function is tightly regulated by its intracellular interacting partners arrestin and the dendritic protein spinophilin. We have previously established the competitive and reciprocal nature of these interacting proteins at the α(2)AR in the context of classic agonist effects, and have shown DMI to be a direct arrestin-biased ligand at the receptor. In the present study, we report that mice deficient in the α(2A)AR subtype lack DMI-induced antidepressant behavioral effects in the FST. As well, mice deficient in arrestin3 lack antidepressant response to DMI, while spinophilin-null mice have enhanced antidepressant response to DMI compared with wild-type controls, indicating that this α(2A)AR-mediated response is reciprocally regulated by arrestin and spinophilin. The characteristic of α(2A)AR-dependence and arrestin3 involvement was shared by the antidepressant effect of the classic α(2)AR agonist clonidine but not the non-tricyclic norepinephrine reuptake inhibitor reboxetine, supporting a model whereby DMI exerts its antidepressant effect through direct engagement of the α(2A)AR and arrestin3. Our results implicate arrestin- and spinophilin-mediated regulation of the α(2A)AR in the pharmacology of the noradrenergic antidepressant DMI, and suggest that manipulation of this mode of receptor regulation may represent a novel and viable therapeutic strategy.

Modulation of stress by imidazoline binding sites: implications for psychiatric disorders

In this review, we present evidence for the involvement of imidazoline binding sites (IBS) in modulating responses to stress, through central control of monoaminergic and hypothalamo-pituitary-adrenal (HPA) axis activity. Pharmacological and physiological evidence is presented for differential effects of different IBS subtypes on serotoninergic and catecholaminergic pathways involved in control of basal and stress-stimulated HPA axis activity. IBS ligands can modulate behavioural and neuroendocrine responses in animal models of stress, depression and anxiety, and a body of evidence exists for alterations in central IBS expression in psychiatric patients, which can be normalised partially or fully by treatment with antidepressants. Dysfunction in monoaminergic systems and the HPA axis under basal and stress-induced activation has been extensively reported in psychiatric illnesses. On the basis of the literature, we suggest a potential therapeutic role for selective IBS ligands in the treatment of depression and anxiety disorders.

Postsynaptic alpha-2 adrenergic receptors are critical for the antidepressant-like effects of desipramine on behavior

The antidepressant desipramine inhibits the reuptake of norepinephrine (NE), leading to activation of both pre- and postsynaptic adrenergic receptors, including alpha-1, alpha-2, beta-1, and beta-2 subtypes. However, it is not clear which adrenergic receptors are involved in mediating its antidepressant effects. Treatment of mice with desipramine (20 mg/kg, i.p.) produced an antidepressant-like effect, as evidenced by decreased immobility in the forced-swim test; this was antagonized by pretreatment with the alpha-2 adrenergic antagonist idazoxan (0.1-2.5 mg/kg, i.p.). Similarly, idazoxan, administered peripherally (0.5-2.5 mg/kg, i.p.) or centrally (1-10 microg, i.c.v.), antagonized the antidepressant-like effect of desipramine in rats responding under a differential-reinforcement-of-low-rate (DRL) 72-s schedule, ie, decreased response rate and increased reinforcement rate. By contrast, pretreatment with the beta-adrenergic antagonists propranolol and CGP-12177 or the alpha-1 adrenergic antagonist prazosin did not alter the antidepressant-like effect of desipramine on DRL behavior. The lack of involvement of beta-adrenergic receptors in mediating the behavioral effects of desipramine was confirmed using knockout lines. In the forced-swim test, the desipramine-induced decrease in immobility was not altered in mice deficient in beta-1, beta-2, or both beta-1 and beta-2 adrenergic receptors. In addition, desipramine (3-30 mg/kg) produced an antidepressant-like effect on behavior under a DRL 36-s schedule in mice deficient in both beta-1 and beta-2 adrenergic receptors. As antagonism of presynaptic alpha-2 adrenergic receptors facilitates NE release, which potentiates the effects of desipramine, the present results suggest that postsynaptic alpha-2 adrenergic receptors play an important role in its antidepressant effects.

Monoaminergic agents modulate antidepressant-like effect caused by diphenyl diselenide in rats

In this study, the antidepressant-like effect caused by diphenyl diselenide on rat forced swimming test (FST) was investigated. The involvement of the monoaminergic system in the antidepressant-like effect was also evaluated. Diphenyl diselenide (0.1-30 mg/kg), given by oral route (p.o.), 30 min earlier, reduced the immobility time in the FST, without accompanying changes in ambulation when assessed in an open field. The anti-immobility effect of diphenyl diselenide (1 mg/kg, p.o.) on the FST was prevented by pretreatment of rats with p-chlorophenylalanine methyl ester (PCPA; 100 mg/kg, i.p., an inhibitor of serotonin synthesis, given once a day, for 3 consecutive days), WAY100635 (0.1 mg/kg, s.c., a selective 5-HT(1A) receptor antagonist), ketanserin (1 mg/kg, i.p., a 5-HT(2A)/(2C) receptor antagonist), ondasentron (1 mg/kg, i.p., a 5-HT(3) receptor antagonist), haloperidol (1 mg/kg, i.p., a D(1), D(2) and D(3) receptor antagonist), SCH233390 (0.05 mg/kg, s.c., a D(1) receptor antagonist), sulpiride (50 mg/kg, i.p., a D(2) receptor antagonist), prazosin (1 mg/kg, i.p., an alpha(1)-adrenoceptor antagonist), yohimbine (1 mg/kg, i.p., an alpha(2)-adrenoceptor antagonist). However, the anti-immobility effect caused by diphenyl diselenide (1 mg/kg, p.o.) on the FST was not affected by pretreatment with propanolol (2 mg/kg, i.p., a beta-adrenoceptor antagonist). Furthermore, monoamine oxidase (MAO) activity was inhibited (39%) in the animals treated with diphenyl diselenide (30 mg/kg, p.o.) when compared to the control group. Taken together these data demonstrated that the antidepressant-like effect caused by diphenyl diselenide seems to be mediated by involvement of the central monoaminergic system.

Evidence for imidazoline receptors involvement in the agmatine antidepressant-like effect in the forced swimming test

This study investigated the involvement of the imidazoline receptors in the antidepressant-like effect of agmatine in the forced swimming test. The antidepressant-like effects of agmatine (10 mg/kg, i.p.) in the forced swimming test was blocked by pretreatment of mice with efaroxan (1 mg/kg, i.p., an imidazoline I1/alpha2-adrenoceptor antagonist), idazoxan (0.06 mg/kg, i.p., an imidazoline I2/alpha2-adrenoceptor antagonist) and antazoline (5 mg/kg, i.p., a ligand with high affinity for the I2 receptor). A subeffective dose of agmatine (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with clonidine (0.06 mg/kg, i.p, an imidazoline I1/alpha2-adrenoceptor agonist), moxonidine (0.5 mg/kg, i.p., an imidazoline I1/alpha2-adrenoceptor agonist), antazoline (1 mg/kg, i.p.) and MK-801 (0.001 mg/kg, i.p., a non-competitive NMDA receptor antagonist), but not with efaroxan (1 mg/kg, i.p.) and idazoxan (0.06 mg/kg, i.p.). Pretreatment of mice with yohimbine (1 mg/kg, i.p., an alpha2-adrenoceptor antagonist) blocked the synergistic antidepressant-like effect of agmatine (0.001 mg/kg, i.p.) with clonidine (0.06 mg/kg, i.p). A subeffective dose of MK-801 (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with antazoline (5 mg/kg, i.p.), but not with efaroxan (1 mg/kg, i.p.) or idazoxan (0.06 mg/kg, i.p.). In conclusion, this study suggests that the anti-immobility effect of agmatine in the forced swimming test is dependent on its interaction with imidazoline I1 and I2 receptors.

Antidepressant-like effect of lamotrigine in the mouse forced swimming test: evidence for the involvement of the noradrenergic system

Lamotrigine is an anticonvulsant drug that is also effective in the treatment of mood disorders, especially bipolar disorder. However, few studies have been conducted in animal models of depression to evaluate its mechanism of action. The present study investigated the effect of lamotrigine in the forced swimming test in mice and the involvement of the noradrenergic system in this effect. Lamotrigine (20-30 mg/kg, i.p.) decreased the immobility time in the forced swimming test and the number of crossings in the open-field test. In addition, the pretreatment of mice with the inhibitor of the enzyme tyrosine hydroxylase, alpha-methyl-p-tyrosine (100 or 250 mg/kg), prevented the antidepressant-like effect of lamotrigine (30 mg/kg, i.p.) in the forced swimming test. Besides that, the pretreatment of mice with prazosin (1 mg/kg, i.p., an alpha1-adrenoceptor antagonist) or yohimbine (1 mg/kg, i.p., an alpha2-adrenoceptor antagonist) also prevented the anti-immobility effect of lamotrigine (30 mg/kg, i.p.). Moreover, the administration of subeffective doses of phenylephrine (5 mg/kg, i.p., an alpha1-adrenoceptor agonist) or clonidine (0.06 mg/kg, i.p., an alpha2-adrenoceptor agonist) was able to potentiate the action of a subeffective dose of lamotrigine (10 mg/kg, i.p.) in the forced swimming test. Thus, the present study suggests that the antidepressant-like effect of lamotrigine in the forced swimming test is related to the noradrenergic system, likely due to an activation of alpha1- and alpha2-postsynaptic adrenoceptors.

Antidepressant-like and antinociceptive-like actions of 4-(4′-chlorophenyl)-6-(4″-methylphenyl)-2-hydrazinepyrimidine Mannich base in mice

This study investigated the possible antidepressant and antinociceptive action of CPMPH Mannich base, as well as the involvement of serotonergic, dopaminergic, noradrenergic and opioid systems and the L-arginine-nitric oxide pathway in the antidepressant-like effect of CPMPH in the forced swimming test (FST) in mice. The immobility time in the FST was significantly reduced by CPMPH (0.1-10 mg/kg, i.p.), without accompanying changes in the ambulation in an open-field. CPMPH at high doses (i.p. or s.c. routes) produced a significant inhibition of acetic acid-induced writhing. The antidepressant-like effect of CPMPH (1 mg/kg, i.p.) in the FST was prevented by pre-treatment of mice with methysergide (2 mg/kg, i.p., a non-selective serotonin receptor antagonist), sulpiride (32 mg/kg, i.p., a D2 receptor antagonist) or yohimbine (1 mg/kg, i.p., an alpha2-adrenoceptor antagonist). In contrast, the antidepressant-like effect of CPMPH was not affected by pre-treatment (i.p.) with naloxone (1 mg/kg, a non-selective opioid receptor antagonist) or L-arginine (750 mg/kg, a nitric oxide precursor). The results demonstrate that CPMPH had an antidepressant-like action that appears to be mediated through its interaction with serotonergic, dopaminergic and noradrenergic systems.

An antidepressant mechanism of desipramine is to decrease tumor necrosis factor-alpha production culminating in increases in noradrenergic neurotransmission

The monoamine theory of depression proposes decreased bioavailability of monoamines, such as norepinephrine (NE), as the underlying cause of depression. Thus, the antidepressant efficacy of NE-reuptake inhibitors such as desipramine is attributed to increases in synaptic concentrations of NE. The time difference between inhibition of reuptake and therapeutic efficacy, however, argues against this being the primary mechanism. If desipramine elicits its therapeutic efficacy by increasing NE release, in turn, increasing activation of the alpha(2)-adrenergic autoinhibitory receptor, then mimicking this increase with an exogenous agonist (clonidine) should support or even enhance the efficacy of the antidepressant. Intriguingly, simultaneous administration of clonidine with desipramine prevented the cellular and behavioral effects elicited by desipramine alone, in both acute and chronic administration paradigms. These results suggest the involvement of additional factor(s) in the mechanism of antidepressant action of this drug. Desipramine administration results in a virtual ablation of neuron-derived tumor necrosis factor-alpha (TNF), thus implicating an essential role of TNF in the therapeutic efficacy of this antidepressant. Additionally, following chronic administration of desipramine, TNF-regulation of NE release is transformed, from inhibition to facilitation. Here, we demonstrate that a transformation in TNF-regulation of NE release in the brain is a key element in the efficacy of this antidepressant. Interestingly, an increase in neurotransmission prior to the antidepressant's effect on TNF production prevents the efficacy of the antidepressant drug. Thus, the efficacy of desipramine is due to decreased levels of TNF in the brain induced by this drug, ultimately modifying noradrenergic neurotransmission.

Sleep disturbances in depression and the effects of antidepressants

Depressed patients often report sleep problems, which usually include difficulties with initiation and maintenance of sleep, as well as poor subjective quality of sleep. Such reports are confirmed by objective analysis of depressed patients' sleep through polysomnography, although there is no exact correspondence between subjective and objective measurements. In the present paper, we discuss some methodological problems related to the subjective estimates of sleep. Further, we review the differential effects of the various classes of antidepressants on subjective sleep parameters, as well as on sleep onset latency, continuity of sleep, sleep efficiency and rapid eye movement (REM) sleep verified with sleep recordings. Finally, we discuss the attempts to use these and other indices, such as delta sleep ratio (DSR), as signposts of the course of the illness, and predictors of response to treatment.

Evidence for serotonin receptor subtypes involvement in agmatine antidepressant like-effect in the mouse forced swimming test

This study investigated the involvement of 5-HT(1) and 5-HT(2) receptors in the antidepressant-like effect of agmatine in the mouse forced swimming test (FST). Pretreatment with p-chlorophenylalanine methyl ester (PCPA; 100 mg/kg, intraperitoneally (i.p.), an inhibitor of serotonin synthesis, for 4 consecutive days), methysergide (5 mg/kg, i.p., a serotonin (5-HT) antagonist), pindolol (32 mg/kg, i.p., a 5-HT(1A/1B) receptor/beta-adrenoceptor antagonist), N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridynyl)cyclohexanecarboxamide (WAY 100635; 0.3 mg/kg, subcutaneously (s.c.), a selective 5-HT(1A) receptor antagonist), 1-(2-methoxyphenyl)-4[-(2-phthalimido)butyl]piperazine) (NAN-190; 0.5 mg/kg, i.p., a 5-HT(1A) receptor antagonist), 1-(2-(1-pyrrolyl)-phenoxy)-3-isopropylamino-2-propanol (isamoltane; 2.5 mg/kg, i.p., a 5-HT(1B) receptor antagonist), cyproheptadine (3 mg/kg, i.p., a 5-HT(2) antagonist) or ketanserin (5 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist), but not with propranolol (2 mg/kg, i.p., a beta-adrenoceptor antagonist), prevented the effect of agmatine (10 mg/kg, i.p.) in the FST. A subeffective dose of agmatine (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with pindolol (32 mg/kg), NAN-190 (0.5 mg/kg, i.p.), WAY 100635 (0.03 mg/kg, s.c.), (+)-8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT; 0.01 mg/kg, i.p., a 5-HT(1A) receptor agonist), R(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI; 1 mg/kg, i.p., a preferential 5-HT(2A) receptor agonist), or fluoxetine (10 mg/kg, i.p., a selective serotonin reuptake inhibitor, SSRI) but not with isamoltane (2.5 mg/kg, i.p.), ritanserin (4 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist) or ketanserin (5 mg/kg, i.p.). Taken together, the results firstly demonstrate that agmatine antidepressant-like effects in the FST seem to be mediated, at least in part, by an interaction with 5-HT(1A/1B) and 5-HT(2) receptors.

Behavioral, neuroendocrine and neurochemical effects of the imidazoline I2 receptor selective ligand BU224 in naive rats and rats exposed to the stress of the forced swim test

RATIONALE

There is evidence for alterations in imidazoline(2) (I(2)) receptor density in depressed patients. Selective I(2) receptor ligands modulate central monoamine levels and activate the hypothalamo-pituitary-adrenal (HPA) axis and may have potential as antidepressants.

OBJECTIVES

To study the behavioral effects of the selective I(2) receptor ligand BU224 in the rat forced swim test (FST) and its effects on the HPA axis and central monoaminergic responses.

METHODS

Rats received saline or BU224 (10 mg/kg IP) 24, 18 and 1 h prior to 15 min exposure to the FST. Saline- and BU224-treated non-stressed groups were included. Time spent immobile, struggling and swimming calmly was measured. Plasma adrenocorticotrophic hormone (ACTH) and corticosterone levels 90 min post-BU224 were measured in addition to tissue levels of monoamines and metabolites in the frontal cortex, hippocampus and hypothalamus.

RESULTS

Administration of BU224 significantly reduced immobility and increased mild swimming without affecting struggling. Exposure to the FST significantly increased plasma ACTH and corticosterone levels. BU224 administration also increased ACTH and potentiated the ACTH response to FST with no effect on corticosterone. BU224 administration significantly increased frontal cortex 5-hydroxytryptamine (5-HT) levels and decreased 5-HT turnover in the frontal cortex and hypothalamus of rats exposed to FST. In non-stressed rats, BU224 decreased 5-HT turnover in the hippocampus and hypothalamus and decreased norepinephrine turnover in the frontal cortex.

CONCLUSIONS

The selective I(2) receptor ligand BU224 reduces immobility of rats in the FST, indicative of antidepressant-like activity. This effect is accompanied by alterations in HPA axis and central monoaminergic activity.

Agmatine produces antidepressant-like effects in two models of depression in mice

Agmatine produces an antidepressant-like effect when assessed in the forced swimming test (FST) and in the tail suspension test (TST) in mice (dose range 0.01-50 mg/kg, i.p.), without accompanying changes in ambulation in an open-field. I.c.v. injection of agmatine (1-100 nmol/site) also reduced the immobility time in the FST. Agmatine significantly enhanced the anti-immobility effect of imipramine, but did not affect that of MK-801. The anti-immobility effect of agmatine assessed in the FST was not affected by pre-treatment with prazosin. In contrast, agmatine's antidepressant-like effect was completely prevented by pre-treatment of animals with yohimbine, GMP or L-arginine. Taken together these data demonstrate that agmatine elicited a significant antidepressant-like effect through a mechanism that seems to involve an interaction with NMDA receptors, the L-arginine-nitric oxide pathway and alpha2-adrenoceptors.